How to compare value inside array in magento 2 join query?

$this->getSelect()->join(
["address" => "sales_order_address"],
'main_table.entity_id = address.parent_id AND address.address_type = "shipping"',
array('postcode')
)->joinLeft(
["so" => "sales_order_tax"],
'main_table.entity_id = so.order_id',
array('sgst' => 'address.postcode' == 600020 ? new Zend_Db_Expr('SUM(amount)/2') : new Zend_Db_Expr(0),'cgst' => 'address.postcode' == 600020 ? new Zend_Db_Expr('SUM(amount)/2') : new Zend_Db_Expr(0),'igst' => 'address.postcode' == 600020 ? new Zend_Db_Expr('SUM(amount)') : new Zend_Db_Expr(0),'amount' => new Zend_Db_Expr('SUM(amount)'))
)
->group('so.order_id');

I need to compare the postcode in the shipping address and based on that i need to set the sgst, cgst and igst value in the sales order grid. what is the correct syntax to compare.